Electrically conductive building material

ABSTRACT

An electrically conductive material used for shielding against electromagnetic waves and comprising a binder, a mixture of graphite and amorphous carbon, and sand. It has been shown that this mixture has enhanced shielding properties, particularly in the 100 MHZ range. The graphite/carbon mixture preferably makes up 25-75% of the total weight of the material. The binder may be cementitious. Said material may also be used in conductive floors, electrical heating equipment, light conductors, heating elements, ground connections, resistors, capacitors and antennas.

FIELD OF THE INVENTION

The present invention concerns an electrically conductive buildingmaterial.

PRIOR ART

It is known so far to use building materials as electrically conductiveshield against electromagnetic radiations. GB patent 1 424 162 discloseselectrically conducting coatings based on cement containing dispersedgraphite applied on a reinforcing grid for a shield coating which cutsfrequencies between 20 KHz to 50 KHz.

In the FR-A-2 216 244 are described coatings based on cement and twodispersed carbons of different granulometry for electrically conductivematerials of structure such as grounding connections, antistatic floorsand walls cutting radio frequencies.

In the JP-A-63 196 672, one has proposed a component for fabricatingelectrical resistors and electrical circuits comprising an organicbinder, an amorphous carbon, graphite in the form of whiskers andsilica.

In the EP-A-032 375, one has described an electrically conductiveelastomer comprising a copolyetherester and a mix of carbon and graphiteintended for use in electronic equipment and office machines.

However, the attenuation performances of known electrically conductiveshields are not linear in a range of frequencies. Some frequency rangesare more important for certain applications than for other, for examplea frequency range which is particularly important for safety shieldingare the frequencies between 1 MHz to 10 GHz. Nevertheless, theattenuation performances of most materials are relatively weak around100 MHz. Since the attenuation performance normally presented is a meanvalue on a large frequency band, the deficiency of attenuationperformances in a given particularly important band frequency, forexample in the region of 100 MHz is often not apparent.

Carbon is allotropic and is found widely in its elementary crystallineor amorphous solid forms. It is found in coke in an amorphous form andexamples crystalline forms are graphite and diamond. In differentapplications, one has used either amorphous carbon or graphite.

SUMMARY OF THE INVENTION

The aim of the present invention is:

1. to propose an electrically conductive building material for cuttingthe electromagnetic radiations and TEMPEST (designation used by NATOconcerning non-intentional radiations phenomena resulting fromcommunication and data treatment systems) et more particularly in therange of frequencies comprised between 1 MHz and 10 GHz.

2. to propose electrically conductive building material for theconstruction of antistatic floors, intrusion sensitive barriers, heatingelements, grounding elements, large capacitors, antennas.

The material according to the present invention is characterized by thefact that it comprises a mixture of graphite and amorphous carbon andsand and in that the binder comprises polyamide resins or polycarbonateresins and a cement-based material.

The advantage of the material according to the present invention is thefact that the simultaneous use of amorphous carbon and graphite producesan unexpected synergy effect between them which improves the materialefficiency.

The examples of binders for the material according to the inventioncomprise either ordinary Portland cement or pulverized fuel ashes orquick-taking cement or a combination of these elements. The relativeratio of binder, sand and graphite and amorphous carbon in the materialaffects as much its shielding properties as its physical properties.High concentrations of carbon in the mixture result in a fragile andbrittle material which is difficult to work and which necessitates asupport, for example taken in sandwich between two plaster sheets. Onthe other hand, if the ratio of carbon in the mixture is lower thanabout 20% of the total weight of the material, then the shieldingefficacy decreases. The preferred graphite/amorphous carbon ratio withrespect to the total weight of the material is between 25% and 75%, andpreferably between 35% and 65%.

The percentage of graphite in the mix graphite/amorphous carbon isbetween 10% and 95% with respect to the weight of this mixture accordingto the planned use of the material. For example in a material in whichgraphite represents 10% of the mix graphite/amorphous carbon, aresistivity of about 0.35 Ohms m is obtained. With a percentage of 50%,the resistivity is 0.024 Ohms m whereas with 95%, the resistivity isabout 0.005 Ohms m. Thus, the higher the graphite percentage, the higherthe required conductivity and the material can be used as anelectromagnetic shield against high frequencies. Conversely, a lowgraphite percentage increases the resistivity which can be useful whenthe material is to be used as a heating element. A high percentage ofamorphous carbon is also useful for reinforcing the material which isnecessary for some applications, when the material is subject to highstress, for example when it is used for floor construction. On the otherhand, in some applications, for example in alarm systems, the materialmust be highly conductive and brittle, in this case the maximalpercentages of graphite are necessary.

It is also important for the performance of the material that the sizeof the graphite particles does not exceed 10 μm and that of theamorphous carbon particles does not exceed 1 mm. It is preferable thatthe size of the sand particles is superior to that of one or the otherof the amorphous carbon or graphite particles. The amorphous carbonpresent in the material as calcined coke or any ashes containingamorphous carbon resulting from the combustion of an organic component.

The material can be improved by introduction of pentavalent andtrivalent substances in different ratios but not exceeding 1% of thetotal weight of the amorphous carbon. Introduction of these additives isuseful for improving the material conductivity by doping thesemi-conductors being in the material. Examples of pentavalentsubstances are among others phosphorus, antimony and arsenic and astrivalent substances gallium and indium.

The material can be completed with other components such asplasticizers, water-repellants such as silicon ether, barium sulfate,zinc and aluminum. The presence of each of said additives can beparticularly advantageous if a shield against high frequencies isnecessary. For example, barium ferrite is particularly efficient forattenuation of frequencies beyond 5 GHz, aluminum improves structuralproperties as much as functional properties (material shielding). Theadding of water-repellants is particularly efficient when the materialis used for making coatings or ceilings.

The invention also concerns a process of fabricating the materialaccording to which a binder or several binders are mixed with a mixturepreviously prepared and containing all the other components.

Preferably, sand is firstly mixed with graphite only, and the thismixture is mixed with amorphous carbon. It appears that in this way, itcan be ensured that the sand particles are effectively covered with thegraphite particles before any other mix, which improves the shieldingproperties of the material.

The aim of the invention, with respect to the attenuation ofelectromagnetic radiation and in particular against TEMPEST radiations,is achieved by the absorption and reflection phenomena. The attenuationis the sum of the capacity of the material to absorb and reflectsignals. This capacity results from the material conductivity as much asfrom the material permeability and porosity after being mixed withwater. Until now, a permeability of 1.26 of the material with respect tocopper has been obtained and a resistivity of 0.024 Ohms m. This isobtained in a mix containing no more than 50% of amorphous carbon andgraphite with respect to the total weight of the material. Amorphouscarbon acts as a semiconductor for increasing the overall conductivityof the material. This effect is then increased by the inclusion oftrivalent or pentavalent substances, which are useful for doping thesemiconductivity of the amorphous carbon and thus increase itsconductivity. The amorphous carbon increases the permeability of thematerial and improves its structural properties. The inclusion ofamorphous carbon improves the efficiency of the material as anelectromagnetic shield.

The material according to the invention can be used principally in threeways:

1. as a coating or mortar comprising the material according to theinvention with about 5% of water with respect to the total weight of thecement-based components;

2. as a sandwich-shaped construction panel comprising two supportsheets, for example plaster, a mix with the building material and waterin a proportion of about 5% of water with respect to the total weight ofthe cement-based components;

3. as a building block comprising the material of the invention alsomixed with water in about 5% of the total weight of the material moldedin blocks of different sizes, such blocks being subjected to compressivestress when fabricated, which improves conductivity by reducing theporosity.

One or the other of these three forms of material according to theinvention can be used in a shielded room or a building againstelectromagnetic radiations by using the coating on the walls, ceilings,floors inside or outside, or by fixing panels to said elements or byconstructing and covering a room or a building with building elementswhich have to be cemented together by using also the building material.

In order to improve the structural properties and improve the shielding,a grid with holes which are not larger than 12 mm in diameter, can alsobe used before applying the panel or the coating. For example by fixinga metal grid having holes of 2,5 mm of opening before the application ofa coating, one improves the attenuation of about 10 dB for frequenciesof about 100 MHz.

The electrically conductive shield, as produced by using the materialaccording to the invention, can be incorporated in the structure ofbuildings or offices, when it is useful, for example, as a shield forattenuating electromagnetic radiations.

In this manner, computer installations can be protected against externalinfluences associated with TEMPEST phenomenon by obtaining anattenuation of at least 30 dB by reflection and absorption offrequencies in the range from 1 MHz to 10 GHz. Other embodiments of theelectrically conductive material, according to the invention allow touse it for the construction of an antistatic floor, light conductors,grounding connections, antennas, resistors, capacitors.

The material according to the invention being electrically conductive,the walls of a room which have been coated with this material arerendered electrically conductive. This can be exploited for realizing acommunication or an alarm system. For example, a room can be alarmprotected by coating the walls with a plaster using the materialaccording to the invention and by applying a voltage. If the walltreated in this way is broken by intrusion, the changes in the voltagecan be detected and used for activating an alarm system. Theconductivity of this material allows to use it as an antenna, the coatedwall acting as a detector, for example for radio signals. Anotherapplication could also be to use this material for building very largecapacitors.

In addition, the material according to the invention presents aresistance and if a current flows through a material, its temperaturerises so that it can be used as a heating element. The power used formaintaining the heating at a stable level is reduced since the materialpresents a negative resistance coefficient increasing with thetemperature thus allowing to reduce the heating cost.

The material previously described has no magnetic properties since it isnot able to retain a charge.

According to another embodiment, one can provide the material withmagnetic properties by adding to the binder, sand, the amorphous carbon,the graphite of the magnetic or magnetizable metallic molecules.

Thus, in a work constructed with such a material, the electrical currentflows through the material, the charge is retained and when the currentsupply is stopped, a discharge in the form of an electrical current isproduced. In the same way, if the material is introduced in a magneticfield, it charges itself electrically.

According to another embodiment, the magnetic metallic molecules areferric molecules.

According to still another embodiment, the metallic molecules aremagnetic salts with the main advantage that when the material is wet orplaced into water, it charges itself up to a certain level, i.e. itbehaves like an electrical battery which can be discharged afterwards.When the material dries, the charge decreases down to a certain level.

The magnetic properties of this material allow to attenuate low and highfrequencies. An interesting application of such shield is the fact thatthe low frequency electromagnetic fields generated, for example, by highvoltage energy transfer cables can be attenuated. This application isimportant since it has been discovered that such magnetic fields areharmful for human health.

The substantial increase of the electrical charge, when this material iswet, allows the user to use the material as a protection membrane on theoutside of a container containing dangerous chemical products since aleak can be detected very quickly by an increase of the electricalcharge which is permanently measured by an adequate system.

An other application of such material could be the detection ofdangerous cracks or fractures in the walls of underwater tunnels orhydroelectric dams, or bridges.

What is claimed is:
 1. Electrically conductive building materialcomprising at least one binder, wherein said material comprises amixture of graphite particles having a particle size not greater thanabout 10 microns, amorphous carbon, and sand wherein the bindercomprises at least one of the substances selected from the groupconsisting of a polyamide resin, a polycarbonate resin, and acement-based material.
 2. Electrically conductive building materialaccording to claim 1, wherein the weight of the mixture of graphite andamorphous carbon represents between 25% and 75% of the total weight ofthe material.
 3. Electrically conductive building material according toclaim 1, wherein the weight of graphite comprises between 10% and 95% ofthe weight of the mixture of graphite and amorphous carbon. 4.Electrically conductive building material according to claim 1, whereinthe weight of sand represents 20% to 50% of the weight of the mixture ofsand and binder.
 5. Electrically conductive building material accordingto claim 1, wherein the amorphous carbon comprises at least one of thesubstances selected from the group consisting of calcined coke and ashescontaining amorphous carbon produced by combustion of an organiccomponent.
 6. Electrically conductive building material according toclaim 1, wherein said material further comprises at least one of thesubstances selected from the group consisting of trivalent andpentavalent substances in a maximal proportion of 1% of the total weightof amorphous carbon.
 7. Electrically conductive building materialaccording to claim 1, wherein the size of the amorphous carbon particlesis smaller than 1 mm, whereas the sand particles are of a larger sizethan the graphite particles, or than the amorphous carbon particles. 8.Electrically conductive building material according to claim 1, whereinthe material comprises water representing about 50% of the total weightof the cement-based materials.
 9. A construction panel comprising thematerial according to claim 8 sandwich formed between two supportsheets.
 10. A construction block, formed by molding the material ofclaim 8 wherein said block is compressed for improving the conductivityby reducing its porosity.
 11. Process for producing the materialaccording to claim 1, characterized by the fact that the binder or theseveral binders is introduced in the mix after the other components havebeen mixed.
 12. Process for producing the material according to claim 1,wherein the sand is firstly mixed with graphite to form a graphite/sandmixture and then mixed with amorphous carbon and the possible trivalentand pentavalent substances.
 13. A method of electroconductivelyshielding a building against electromagnetic radiations by placing ashield between the exterior of the building and the source ofelectromagnetic radiations, said shield selected from the groupconsisting of the material according to claim 8; the construction panelaccording to claim 9 and the construction block according to claim 10.14. The method according to claim 13, characterized by the fact thatsaid shield further comprises a metal grid with a mesh size not largerthan 12 mm.
 15. Electrically conductive building material according toclaim 1, wherein the material further comprises magnetic or magnetizablemetallic molecules.
 16. Electrically conductive building materialaccording to claim 15 wherein said molecules are ferrous molecules. 17.Electrically conductive building material according to claim 15 whereinsaid molecules are magnetic salts.